The present invention relates to a method and a system for starting up a turbomachine of an aircraft, in particular of a transport airplane.
The invention applies to a start-up system of the type comprising:                a ventilation source, in particular an axillary power unit (APU), which is able to produce an airflow and which comprises a compressor fed by an air intake circuit regulated by at least one controllable inlet valve; and        a start-up turbine, which is fed by said ventilation source via a pneumatic connection and which is able to produce, when subjected to a fluid flow, a mechanical force making it possible to turn a rotor of the engine (turbomachine) of the aircraft for the purpose of the start-up.        
The engine is then switched on, in the conventional manner, after injection of fuel in the appropriate elements of the engine, which have been rotated by the rotor.
It is known that, during an operating cycle of an airplane, in particular of a short-haul or medium-haul transport airplane, the airplane lands at an airport generally so as to allow the passengers to disembark and so as to allow other passengers to embark before departing for another destination. This change of passengers generally lasts between thirty minutes and one hour and thirty minutes.
During this time, the engines of the airplane, which are stopped (for reasons of safety), remain hot and do not have time to cool completely. It is estimated that, generally, an engine is cold after having been stopped for approximately one hour and thirty minutes.
Thus, during the waiting time, a thermal gradient at the rotor or the rotors of the engine causes the expansion of some blades and/or of the rotor or rotors, which deform (causing a reduction of the axial or diametrical play with respect to the normal axis of rotation of the blading, an expansion of the blades, etc.).
During the restart, if the engine has not had time to cool sufficiently, the ends of some blades of the rotor are at risk of rubbing against the casing, or a blading will be slightly offset from its axis of rotation. This phenomenon, referred to as “bowed rotor”, lasts until the temperature between the bladings becomes uniform due to the operation (rotation and reduction of internal thermal gradients, realignment of the turning parts) of the engine.
The rotation of incorrectly aligned turning parts of the engine leads to vibrations due to the imbalance. These vibrations become particularly detrimental for some values of speed of rotation of the turning parts, that is to say the speed N2 of the compression rotor (high-pressure model) of a turbofan. This is particularly the case for values of speed N2 approaching critical frequencies of the engine, due to the specific amplification of rotor/stator friction in the vicinity of the vibratory resonances.
The aforementioned phenomenon causes a significant increase of the fuel consumption due to a loss of aerodynamic efficacy in particular of different elements of the engine, moreover irreversibly (wear of the ends of the blades).